Vertical drainage drying bed for waste sludge and an intensified method of treating wastewater

ABSTRACT

A drying bed for waste sludge in a wastewater treatment facility having a vertical drainage riser extending vertically from a subnatant discharge pipe near the bottom of the drying bed. A sludge inlet at an end of the drying bed communicates with a source of watery sludge. Water communicates through a mesh screen that wraps the vertical drainage riser and through openings in the vertical drainage riser to the subnatant discharge pipe. A valve is selectively opened to control the flow rate of the water from the subnatant discharge pipe. A method of dewatering waste sludge is disclosed. The wastewater treatment facility provides a mixing/balancing chamber for initial mixing of activated return sludge with influent wastewater for aggressive assimilation of wastes and an improved airlift for removal of suspended solids in the wastewater. The airlift comprises a solids evacuation pipe having a lower open end for receiving solids, an upper open end for exiting solids, and a midsection defining a series of holes. A sealed manifold encloses the midsection of the solids evacuation pipe. An air supply line connects at one end to a supply of pressurized air and the other end connects to the interior of the manifold. The air, being introduced through the holes into the solids evacuation pipe, produces lift for causing the solids to enter the lower open end of the solids evacuation pipe and to exit the upper open end of the solids evacuation pipe.

This is a division of co-pending application Ser. No. 08/285,053 filedAug. 3, 1994.

TECHNICAL FIELD

The present invention relates to wastewater treatment facilities. Moreparticularly, the present invention relates to a drying bed fordewatering waste sludge in a wastewater treatment facility and a methodof treating wastewater which reduces the amount of sludge produced perunit of wastewater treated.

BACKGROUND OF THE INVENTION

Wastewater treatment by contact with activated sludge is well-known. Ina typical wastewater treatment plant, the influent passes through aheadworks for screening and grit removal and then a series of treatmentprocesses. Screening removes roots, rags, cans and large debris. Grit isremoved in a quiescent section of the headworks. Preaeration freshenswastewater and helps remove oil. This primary treatment of sedimentationand flotation removes settleable and floatable materials. A secondarytreatment of blending the raw influent wastewater with return activatedsludge biologically stabilizes wastewater by removing the suspended anddissolved solids. This activated sludge contains microorganisms whichassimilate the waste materials. Disinfection kills pathogenic organismsin the clarified wastewater. The resulting effluent is then generallydischarged to surface waters.

Upon entering the secondary treatment process, the raw wastewater ismixed in a first aeration tank with return activated sludge whichtypically comprises relatively high concentrations of microorganisms.The return activated sludge comes from a secondary clarifier, asdiscussed below. The mixture is aerated and agitated in a series ofaeration tanks to facilitate the growth of the sludge. The addition ofair induces the growth of the microorganisms living in the sludge. Themicroorganism, such as bacteria, fungi, and protozoa, feed on the rawwastewater to reduce and decompose the wastes in the wastewater. Theaeration process is approximately 24 hours.

Following aeration, the mixture is allowed to settle in the secondaryclarifier. The microorganisms and waste collect into larger clumps ofmaterial known as floc. The activated sludge floc separates from thewater by gravitational force due to its higher specific gravity andsettles on the bottom of the secondary clarifier. The water on thesurface is removed to a disinfecting tank. This disinfected water is theplant effluent and is ready for disposal by dilution or direct dischargeto surface waters. The activated sludge from the bottom of the secondaryclarifier, known as return activated sludge, is pumped to the firstaeration tank for mixture with the raw influent wastewater, thuscompleting the cycle.

Activated sludge is typically measured in terms of biochemical oxygendemand (BOD) in milligrams per liter (mg/l), which is the strength ofthe wastewater and primary food source for the microorganisms, and totalsuspended solids (TSS). Domestic raw wastewater typically is 250 mg/lBOD and 200 mg/l TSS. The return activated sludge solids concentrationis typically between 2000-6000 mg/l TSS. The plant effluent is typically10 mg/l for both TSS and BOD. The activated sludge continues to grow byassimilating waste products as it passes through the process.

Sludge dewatering may be accomplished by several methods, such as dryingbeds, sludge lagoons, withdrawal of wet sludge to land as topicalfertilizer, and mechanical apparatus such as vacuum filters andcentrifuges. Small capacity wastewater plants typically use drying bedsfor dewatering sludge. The drying process occurs by evaporation andpercolation of the water from the sludge. Typical drying beds are 15 to18 inches in height and have a drainage system under the bottom of thebed. The drainage system typically has a layer of coarse crushed rock, alayer of gravel, a layer of pea gravel and a cover layer of 6 to 8inches of sand. The sludge is applied on top of the sand to a depth ofapproximately 12 to 14 inches. The drying time in warm weather istypically 4 weeks. Rain or other precipitation increases the dryingperiod. Dried sludge is removed from the bed manually or with heavyequipment and may be used as a low grade fertilizer. The drainage waterfrom the drying beds is typically returned to the headworks. Thequantity of dried sludge produced is typically 60% of each unit ofinfluent wastewater BOD treated.

Wastewater treatment plants utilizing drying beds have large spacialrequirements in that as waste sludge is produced need to access dryingbeds is required. These plants are extremely susceptible tometeorological conditions. If it rains, the sludge rehydrates andrequires increased drying time to remove the additional water. Withlarge amounts of rain, the sludge will not dry. After drying the sludgeis removed manually for disposal. Sludge is typically removed from thebeds by a shovel.

Thus, a need exists for a wastewater treatment process that does notgenerate excessive sludge and has an effective sludge dewatering systemwhich is not impacted by climate conditions.

SUMMARY OF THE INVENTION

The present invention solves the above-described problems in the priorart by providing an improved drying bed and method for dewatering wastesludge and an improved method of treating wastewater for reducing theamount of sludge produced.

Briefly described, the drying bed of the present invention comprises aholding tank for dewatering waste sludge in a wastewater treatmentfacility. The holding tank includes a bottom surface with a subnatantdischarge pipe. A sludge inlet at an inlet end of the holding tankconnects with a source of waste activated sludge for supply to theholding tank. The subnatant discharge pipe connects to a valve at adischarge end of the holding tank. At least one vertical drainage riserconnects to the discharge pipe and extends upwardly in the holding tank.The vertical drainage riser includes spaced-apart openings forcommunicating water from the waste activated sludge into the verticaldrainage riser. The water, being communicated into the vertical drainageriser and the discharge pipe, flows through the discharge valve at apredetermined rate for dewatering the sludge.

The present invention further provides a method of dewatering wastesludge created in a wastewater treatment facility. Briefly described,the method comprises the steps of filling a drying bed with wasteactivated sludge. The sludge separates from the water in the drying bed,first by sinking to the bottom and then floating to the surface. Thewater then communicates through one of a plurality of openings in avertical drainage riser that extends upwardly from a subnatant dischargepipe disposed near the bottom of the drying bed. A valve connected tothe discharge pipe is opened a predetermined amount for controlling theflow of water through the discharge pipe so that particulates are notcarried by the flow of water through the openings. The water, beingcommunicated into the vertical drainage riser and the discharge pipe,flows through the discharge valve for dewatering the sludge.

The present invention further provides a method of treating wastewaterto reduce the amount of sludge produced per unit of wastewater treated.Briefly described, the method comprises supplying raw influentwastewater and return activated sludge to a mixing/balancing chamber,whereby microorganisms in the sludge aggressively feed on the influentwastewater. The activated sludge is removed by airlift from a lowerportion of a settling chamber in the wastewater treatment facility. Themixture of the raw influent wastewater and the return activated sludgethen flows in sequence through a series of aeration tanks agitated byinjecting air into the mixture, whereby the microorganisms continue togrow and feed on the wastewater. The activated sludge then enter a calmsettling chamber where the sludge and the water separate. Theconcentration of activated sludge is monitored, and upon reaching apredetermined concentration, a portion is transferred by airlift to adrying bed for dewatering the concentrated activated sludge andsubsequent disposal.

The present invention further provides an airlift comprising a solidsevacuation pipe having a lower open end for receiving solids, an upperopen end for exiting solids, and a midsection defining a series ofholes. A sealed manifold encloses the midsection of the solidsevacuation pipe. An air supply line connects at one end to a supply ofpressurized air and the other end connects to the interior of themanifold. The air, being introduced through the holes into the solidsevacuation pipe, produces lift for causing the solids to enter the loweropen end of the solids evacuation pipe and to exit the upper open end ofthe solids evacuation pipe.

The features and advantages of the present invention will becomeapparent from a reading of the following specification, in conjunctionwith the drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a drying bed for dewatering of wastesludge according to the present invention.

FIG. 2 is a perspective view of a portion of the drying bed illustratedin FIG. 1, to show details in a cut-away view.

FIG. 2A is a cross-sectional view of the horizontal flexible pipe inFIG. 2.

FIG. 3 is a plan view of a facility for treating wastewater to reducethe amount of sludge produced per unit of wastewater treated and fordewatering sludge using the drying bed illustrated in FIG. 1.

FIG. 4 is an enlarged view of the secondary clarifier of FIG. 3detailing an airlift of the present invention.

FIG. 5 is a cross-sectional view of a mid-portion of a sludge evacuationpipe within a manifold, which is a component of the airlift of FIG. 4.

DETAILED DESCRIPTION

Referring now in more detail to the drawings, in which like numeralsindicate like parts throughout the several views, FIG. 1 shows aperspective view of a preferred embodiment of a drying bed 10 accordingto the present invention. The drying bed 10 in the illustratedembodiment comprises a concrete basin 12 formed with exterior walls 14and divided into two holding tanks 16 by a wall 18. The drying bed 10 inthe illustrated embodiment is partially below ground level 19. Eachholding tank 16 includes a pair of recessed trenches 20 that extendbetween an inlet side 22 of the tank to a discharge side 24. A bottomfloor 26 of the tank 16 comprises four plates 28 that taper at a smallangle downwardly to a respective upper edge of one of the trenches 20,for inducing water flow to the trenches 20. For example, the plate 28atapers from the exterior wall 14a to the upper edge of the trench 20a.The plates 28b and 28c taper in opposite directions from a ridge 30towards the upper edge of the respective trenches 20a and 20b. The plate28d tapers downwardly from the wall 18 to the trench 20b.

Each trench 20 holds a subnatant discharge pipe 32 which is buried ingravel. The subnatant discharge pipe 32 preferably assembles byconnecting lengths of flexible pipes 36 together with T-connectors 38.In the illustrated embodiment, the pipes 36 are approximately threemeters long. The pipes 36 preferably are perforated with openings, for apurpose discussed below. A vertical drainage riser 40 connects at alower end to the T-connector 38 and extends upwardly from the trench 20.In the illustrated embodiment, the vertical drainage riser is acylindrical pipe. The vertical drainage riser 40 includes a series ofholes 42 for communicating water from waste activated sludge (WAS) inthe drying bed 10 to a discharge pipe, as discussed below. A removablecap 43 closes an upper distal end of the pipe 40.

An end 46 of the subnatant discharge pipe 32 abuts the wall 14 at theinlet side 22 of the tank 16. The end 46 is closed by a cover. An outletend 48 of the subnatant discharge pipe 32 connects to an outlet manifold50 which receives fluid flow from each of the subnatant discharge pipes32. A pipe 52 connects the manifold 50 to a valve 54. Art effluentreturn pipe 56 connects the valve 55 to an input of a wastewatertreatment facility, for a purpose discussed below.

The wall 14 at the discharge end 24 of the tank includes a gate 58. Thegate 58 is removable and sized for permitting entrance into the tank 16by a small front-end loader or other such apparatus for removal of driedsludge.

An inlet manifold 60 connects to a supply of WAS at the inlet side 22 ofthe tank 16. The inlet manifold communicates the WAS from a clarifier inthe wastewater treatment facility to the drying bed 10. Downspouts 62connect the manifold 60 to supply the WAS to the tank 16. Although notillustrated, each downspout 62 can be connected to a valve for selectivesupply of the WAS to one of the tanks 16.

FIG. 2 is a perspective view of a cut-away portion of the drying bed 10for illustrating details of the connection between the subnatantdischarge pipe 32 and the vertical drainage riser 40. The drying bed 10preferably is conventional form-cast concrete on a soil, sand and gravelfoundation generally designated 70. The trench 20 is filled with gravel72 in which the subnatant discharge pipe 32 is embedded. A mesh screen73 lays across and covers the opening of the trench 20. The mesh screen73 is preferably a metal screen having a mesh of about one millimeter. Alayer of sand 75 covers the mesh screen 73. The sand has a depth ofpreferably about 25 millimeters. The pipes 36 connect together with theT-connectors 38. The vertical drainage risers 40 extend upwardly fromthe T-connectors. The vertical drainage risers 40 preferably are rigidand each include a number of spaced-apart openings 42. In a preferredembodiment, a lower portion comprising about one-third the length of thepipe 40 has a first set of openings each with a diameter of sixmillimeters. An upper portion of about two-thirds the length of the pipe40 has a second set of openings each with a diameter of threemillimeters. The openings are spaced apart about thirty-two millimeters.

A filter media 74 mounts inside an upwardly extending leg 76 of theT-connector 38. The filter media 74 in the illustrated embodimentcomprises a mesh screen 78 mounted transverse to a direction of flowthrough the leg 76. The mesh screen 78 supports layers of gravel 80 andof sand 81. The gravel 80 is about 25 millimeters in diameter and has adepth of about 38 millimeters. The sand 81, which is mounted on top ofthe gravel 80, has a depth of about 25 millimeters.

A mesh screen 82 wraps around the vertical drainage riser 40 forfiltering the water entering the vertical drainage riser. The meshscreen 82 is preferably an aluminum or plastic screen having a mesh ofabout 0.5 millimeters. Straps (not illustrated) secure the mesh screen82 around the vertical drainage riser 40, although other connectingmechanisms can be used.

The pipe 36, which is preferably polyvinyl chloride plastic, includesopenings 84 which are spaced apart approximately 20 cm. The openings 84are oriented at about a downward 45 degree angle from a line defined bya longitudinal axis 85 of the pipe 36, as best shown in FIG. 2A. In apreferred embodiment, the pipes 36 are disposed in the gravel at a slopefrom the input end 22 to the discharge end 24 of the drying bed. Theslope is preferably about 20 degrees.

FIG. 3 is a schematic diagram of a wastewater treatment facility 100that uses the drying bed 10 of the present invention. The facility 100includes an influent supply 102 that communicates with chambers 103having bar screens for catching large solids such as sticks, roots, cansand other large debris, and for breaking up dissolvable solids that maybe carried in the wastewater. Bar screens are conventional in the artfor such purposes. A pipe 106 connects the chambers 103 with amixing/balancing chamber 108. An airlift 110, described in more detailbelow, is disposed in the chamber 108. The airlift 110 has a lower openend and connects near the lower end to a supply of pressurized air, suchas a blower 112. The airlift 110 extends upwardly with an open distalend communicating with an aeration tank 114. The aeration tank 114includes air supply pipes 116 that communicate with the blower 112.

A ten centimeter outlet channel 118 at one end of the aeration tank 114has a recessed lip 120 over which mixed liquor flows. The outlet channel118 communicates with a lower portion 122 of a secondary clarifier 124.Another airlift 126, similar in construction to the airlift 110 in themixing/balancing chamber 108, is disposed in the secondary clarifier 124and has an open lower end. The airlift 126 connects near the lower endto the supply of pressurized air 112. An upper open end 128 connects toa diverter 130, such as a valve tree, or the like. The diverter 130connects through a pipe 132 to the mixing/balancing chamber 108 andconnects through a pipe 134 to the input manifold 60 of the drying bed10.

The secondary clarifier 124 includes a discharge launder 136 having arecessed lip 138. Clarified water travels over the lip 138 into a pipe140 that communicates with an tertiary aeration pond 142. The aerationpond 142 includes air pipes 144 for communicating air from the blower112 into the pond. A discharge pipe 146 opens at an upper edge of thepond 142 for receiving water from the pond. The discharge pipe 146communicates with a chlorine contact chamber 147 which receivesdisinfectants from a supply 148. A discharge chamber 150 receives thechemically treated water from the chamber 146 through a pipe 152. Anoutlet 154 from the chamber 150 communicates with surface waters ortemporary holding pond for subsequent discharge to surface waters.

The drying bed 10 includes the discharge manifold 50 as discussed above.The valve 54 controls flow of water from the drying bed through thedischarge pipe 56 which connects the mixing/balancing chamber 108.

FIG. 4 is an enlarged view of the secondary clarifier 124 showingdetails of the airlift 126, as well as the airlift 110. The airlift 126includes an air supply line 200 that is connected at its upper end tothe blower 112 and at its lower end to a manifold 201. The manifold 201is a sealed cylinder having an upper end 202 and a lower end 203. Theair supply line 200 opens into the interior of the manifold 201. Asludge evacuation pipe 204 extends through the upper end 202 and thelower end 203 of the manifold 201. The manifold 201 thereby encloses amidsection 205 of the sludge evacuation pipe 204. The midsection 205contains a series of spaced-apart holes 206. A lower section 207 of thesludge evacuation pipe 204 extends to the bottom of the secondaryclarifier 124 and receives sludge and water through an opening 208 asdiscussed below. An upper section 209 of the sludge evacuation pipe 204extends out of the secondary clarifier 124 and divides into an air port210 and the diverter 130. The sludge evacuation pipe 204 and the lowerend of the air supply line 200 are offset near the sides of the manifold201. A brace 211 connected between the air supply line 200 and thesludge evacuation pipe 204 may be used to provide support for the airsupply line 200.

FIG. 5 shows a cross-section view of a preferred embodiment of themidsection 205 within the manifold 201. The holes 206 are preferably ina semi-circular pattern covering one half of the midsection. There areseven columns of holes 206. Each hole 206 is spaced-apart approximatelythirty degrees. The midsection preferably includes twenty rows of holes206 that are four millimeters in diameter. The manifold 201 has a heightof twenty centimeters such that the holes 206 are enclosed within themanifold.

The drying bed 10 shown in FIG. 1 is used with the wastewater treatmentfacility 100 shown in FIG. 3. Raw wastewater enters the facility 100through the influent supply 102. While in chambers 103, bar screens (notshown) catch large solids and break up dissolvable solid that may becarried in the raw wastewater. The resulting influent travels throughthe pipe 106 to the mixing/balancing chamber 108. The mixing/balancingchamber 108 also receives concentrated activated sludge recovered fromthe secondary clarifier 124. This sludge is conventionally known asreturn activated sludge (RAS). The RAS from the secondary clarifier 124travels through the pipe 132 into the mixing/balancing chamber 108.Also, subnatant water from the drying bed 10 travels through the pipe 56to the mixing/balancing chamber 108.

The microorganisms in the secondary clarifier 124 settle to the bottomprior to transfer to the mixing/balancing chamber 108. Themicroorganisms in the sludge become dormant as they are increasinglydeprived of oxygen and assimilate food. After transfer to themixing/balancing chamber 108, the microorganisms in the RAS becomeaggressively active in feeding on the waste products and nutrients inthe influent and in reproducing. The nutrient content of the materialsin the mixing/balancing chamber 108 is monitored. As necessary,additional nitrogen and phosphorus are added, so that the fluid mixturein the chamber 108 has a controlled nutrient concentration ratio, with apreferred nitrogen ratio of about one percent and a phosphorus ratio ofabout 5 percent. This provides optimum nutrients on which themicroorganisms in the wastewater reproduce.

The environment of the chamber 108 induces rapid growth and increasedfeeding since it is provided with additional nitrogen and phosphorus andthe influent wastewater itself is high in food content for themicroorganisms. The increased microorganism activity prior to enteringthe aeration process accelerates the assimilation of wastes. Moremicroorganisms are available to stabilize the more wastewater. The endresult is a process that produces approximately 10% sludge per unit ofwastewater treated.

The present invention accordingly improves conventional wastewatertreatment facilities in which the RAS is merely communicated to thefirst aeration tank. In conventional facilities, the RAS enters thefirst aeration tank in which the food content of the influent has beensignificantly diluted. As a result conventional process generate about65% sludge per unit of wastewater treated.

The air lift 110 as described above communicates the nutrient-richbalanced wastewater known as mixed liquor from the mixing/balancingchamber 108 to the aeration tank 114. The air flow moves the sludge andwater upwardly through the air lift 110 to the aeration tank 114.

Air pipes 116 communicate with the blower 112 to inject air into themicrobe-active mixed liquor the aeration tank 114. The air providesoxygen for the microorganisms to live. The injection of air into thelower portion of the aeration tank 114 turbulently stirs the mixedliquor in the tank. This thoroughly mixes the microorganisms and thewaste materials and adds oxygen content. The microorganisms ingest thewaste materials in the mixed liquor. The microorganisms grow. The wastematerials also adhere to the microorganisms. The waste andmicroorganisms clump together forming floc.

The mixed liquor flows over the recess lip 120 of the outlet channel 118at one end of the aeration tank 114. The mixed liquor flowing throughthe outlet channel 118 carries the floc clumps of waste materials andmicroorganisms.

The inflow through the channel 118 enters a lower portion of thesecondary clarifier 124. This area in the secondary clarifier is calm.The sludge settles to the lower portion of the secondary clarifier 124.The accumulated sludge is picked up by the air lift 126 forcommunication through the diverter 130 to either the mixing/balancingchamber 108 or to the drying bed 10.

With reference to FIG. 4, the airlift 126 operates by the introductionof air through the air supply line 200. The air enters the manifold 201and flows through the holes 206 of the mid-section 205 of the sludgeevacuation pipe 204. The air bubbles up the sludge evacuation pipe 204to exit at the air port 210. The air flow creates a lift in the sludgeevacuation pipe 204 thereby moving the WAS from the lower portion 122 ofthe secondary clarifier 124 to the diverter 130 for either return to themixing/balancing chamber 108 or dewatering in the drying bed 10.

The airlift 126 of the present invention introduces air at more than onepoint in the sludge evacuation pipe 204. The air flow from the manifold201 through the numerous holes 206 improves the lift for elevating theWAS. Having holes 206 covering one-half of the mid-section 205 minimizesthe separation of solids and liquids thereby enhancing the physicalmovement of the WAS. This particular type of airlift may be employed toremove other types of liquid materials containing suspended solids.

By using the airlift 126, electric pumps are no longer needed. Pumpsstress the microorganisms. The pumps also shearing the microorganisms inthe WAS. This typically kills many of the microorganisms and causesseparation of the solids and liquids. Thus, the WAS is more effectivelymaintained for the reproduction of waste-assimilating microorganisms.

Introduction of air by the airlift 126 also stimulates themicroorganisms after being dormant in the lower portion 122 of thesecondary clarifier 124. Aeration is now provided at a critical stagefor reactivation of the WAS prior to returning to the mixing/balancingchamber 108.

In FIG. 3 the air that communicates from the blower 112 is introducedinto the wastewater treatment process at various points. It is preferredthat all of these points of introduction be on the same level tomaintain balanced air pressure within the process.

The activated sludge in the lower portion 122 of the secondary clarifier124 is selectively sent to the drying bed 10 when the concentration oftotal suspended solid in the activated sludge is between about 6,000 and10,000 milligrams per liter. Otherwise, the activated sludge or RAS iscommunicated to the mixing/balancing chamber 108.

The clarified wastewater in the top of the secondary clarifier 124overflows the recess lip 138 of the discharge launder 136. The clarifiedwater travels through a pipe 140 into a tertiary aeration pond 142. Airpipes 144 communicate air from the blower 112 into the pond 142. Theaerated water overflows into a discharge pipe 146 which communicateswith a chlorine contact chamber 147. Disinfectants such as chlorine, orstabilizers such as lime or chlorine are added to chemically neutralizethe biologically active microorganisms and other pathogens in the water.The treated water is discharged through an outlet 152 to surface waters.In the illustrated embodiment, the treated water passes through adischarge chamber 150 prior to discharge to a river or temporary holdingpond.

As discussed above, activated sludge or WAS is periodically wasted todrying bed 10. The floc, or sludge, in the secondary clarifier 124 issampled to determine TSS concentration. There are a number ofconventional tests for determining concentration, including acentrifuge, mass balance, or settleometer. When the sludge reaches TSSconcentrations of between about 6,000 and 10,000 milligrams per liter,the sludge is diverted to the drying bed 10. Typically, sludge reachesconcentrations of 10,000 milligrams per liter in about seven days.

Prior to introducing the WAS to the drying bed 10, the valve 54 isclosed. The diverter 130 is then opened to communicate the sludgethrough the pipe 134 to the inlet manifold 60. The WAS is deliveredthrough the nozzles 62 to fill the tank 16. During the initial 30minutes the sludge slowly settles to the bottom 28 which formsstratified layers of water and sludge. The active sludge does not massand compact together as does inactive sludge. The settling activatedfloc instead leaves pathways known as capillaries for flow of freewater.

After about 6 to 8 hours, a natural process of denitrification occursand the sludge begins to float to the surface and reverse the stratifiedlayers. The waste sludge is in a top layer floating on the water below.The valve 54 is then opened to provide a controlled flow of water fromthe drying bed. A settleometer (not shown) may be placed on the pipe 52adjacent the valve 54 to observe the denitrification process and theflow of water from the dry bed 10. For the first hour the valve 54 isopened halfway for a rate of flow of about 3.5 millimeters per second.Thereafter, the valve 54 is fully opened to a preferred rate of flow ofabout 7 millimeters per second. The water in the tank 16 communicates bycapillary action through the mesh screen 82 and the holes 42 in thevertical drainage riser 40. This low flow rate by capillary actionfacilitates transfer of the water without a forceful rush that wouldcarry particulate matter through the screen 82. Such forceful carryingof particulate matter through the screen would deposit excess sludgesolids in the subnatant discharge pipe 32 thereby restricting subnatantwater flow. The water travels down the vertical drainage riser 40through the T-coupler 38 to the subnatant discharge pipe 32. Water nearthe bottom of the holding tank 16 flows through the sand 75 and the meshscreen 73 into the trench 20. The sand 75 and the mesh screen 73substantially prevent the gravel 72 from becoming clogged withparticulate matter. Once the water within the trench 20 reaches a levelequivalent to the holes 84 in the subnatant discharge pipe 32, the waterflows into the subnatant discharge pipe through the holes 84. Thesubnatant water in the subnatant discharge pipe 32 then returns throughthe effluent return pipe 56 to the mixing/balancing chamber 108. Theconcentration of the subnatant water is approximately 40 to 70 mg/l TSS.

Additional applications of activated sludge from the secondary clarifier124 to the drying bed 10 can be applied during the drying process whilemaintaining efficient dewatering of the sludge in the bed. This is animprovement over conventional drying beds. For example, the availablevolume in a drying bed may be required to handle excess flows caused bystorm runoff. The limited capacity of the conventional drying bedprevents the treatment facility from handling the excess inflow. In thatevent, raw sewage may be discharged into streams or lakes. The dryingbed 10 of the present invention however can be deep, for example tenfeet deep, as compared to the conventional drying bed of about 14inches, thereby providing additional treatment capacity during stormevents. The deeper drying bed 10 further enhances the dewateringprocess, in that the resulting pressure at the lower portion of a deeperdrying bed will be greater than a shallow drying bed. This facilitatesthe drainage of water through the vertical drainage risers 40 and thedrainage pipes 32.

After the flow of subnatant water from the drying bed 10 slows or stops,the raw sediment is dried for four to seven days under the influence ofsolar radiation until the moisture content is about 60 to 70 percent.The dried sediment is now ready to be removed for disposal. The verticaldrainage risers 40 are removed and backwashed to clean the interior ofthe pipes and the mesh screens 82. In the illustrated embodiment, thegate 58 is opened and a front end loader is employed to scoop up thesediment and deliver it to a truck for disposal. The subnatant dischargepipes 32 are likewise washed, if necessary, as is the floor of theholding tank 16 before another volume of sludge is received fordewatering.

Thus, there has been described a wastewater treatment method with animproved airlift and a drying bed and method having improvements andbenefits over conventional methods and drying beds. The presentinvention eliminates the digester, significantly reduces the volume ofsludge produced per unit of wastewater treated, and reduces the dryingtime of sludge even in periods of precipitation.

The principles, preferred embodiments, and modes of operation of thepresent invention have been described in the foregoing specification.The invention is not to be construed as limited to the particular formsdisclosed because these are regarded as illustrative rather thanrestrictive. Moreover, variations and changes may be made by thoseskilled in the art without departing from the spirit of the invention asdescribed by the following claims.

What is claimed is:
 1. An airlift for communicating accumulated wastesludge from a lower portion of a secondary clarifier of a sewagetreatment facility, comprising:an elongated sludge evacuation pipehaving a lower end open to a sludge settling basin in lower portion of asecondary clarifier of a sewage treatment facility for receivingconcentrated waste activated sludge and water in which the wasteactivated sludge is carried, an upper end for discharging of the wasteactivated sludge and water selectively to a drying bed and a clarifiedwater discharge or to an aeration tank, and a midsection spaced-apartfrom the lower end and defining a series of spaced-apart holes arrangedin a semi-circular side portion of the midsection; a sealed cylindricalmanifold enclosing the midsection of the sludge evacuation pipe; an airsupply line connected at one end to the manifold for communicating airinto the interior of the manifold and for connecting at another end to asupply of pressurized air, whereby the air, being introduced through theholes into an interior side portion of the sludge evacuation pipe,produces a lifting effect within the sludge evacuation pipe forelevating the waste activated sludge from the settling basin in thelower portion of the secondary clarifier while reducing the breakup andseparation of the waste activated sludge into smaller particles in thewater, thereby enhancing the physical treatment of raw sewage in theaeration tank and the settling of the waste activated sludge in thesecondary clarifier during the cyclic processing of the sewage andsludge in the sewage treatment plant.